Which characteristic defines the best-quality star corundum?

The characteristic that defines the best-quality star corundum is semi-transparent. Star corundum, such as star sapphires, is known for its unique asterism, which is the star-like pattern that appears on the surface of the stone. This phenomenon occurs due to the presence of needle-like inclusions, typically made of rutile, that reflect light in a specific way.

While semi-transparent stones can exhibit significant depth and clarity that allows for the observation of the star effect, the presence of inclusions plays a crucial role in enhancing this characteristic. It is these inclusions that create the star, making the semi-transparent quality vital for achieving the desired visual effect.

In contrast, opaque stones do not allow light to pass through, which diminishes the star's visibility and brilliance. Fully transparent stones may not exhibit the same kind of striking star pattern, as the necessary inclusions that contribute to the asterism may be absent. Therefore, semi-transparency offers the ideal balance for showcasing the captivating star phenomenon that defines high-quality star corundum.